Delayed and incomplete reprogramming of chromosome methylation patterns inbovine cloned embryos

Full-term development has now been achieved in several mammalian species by transfer of somatic nuclei into enucleated oocytes [1, 2]. Although a high proportion of such reconstructed embryos can evolve until the blastocyst s tage, only a few percent develop into live offspring, which often exhibit d evelopmental abnormalities [3, 4]. Regulatory epigenetic markers such as DN A methylation are imposed on embryonic cells as normal development proceeds , creating differentiated cell states. Cloned embryos require the erasure o f their somatic epigenetic markers so as to regain a totipotent state [5]. Here we report on differences in the dynamics of chromosome methylation bet ween cloned and normal bovine embryos before implantation. We show that clo ned embryos fail to reproduce distinguishable parental-chromosome methylati on patterns after fusion and maintain their somatic pattern during subseque nt stages, mainly by a highly reduced efficiency of the passive demethylati on process. Surprisingly, chromosomes appear constantly undermethylated on euchromatin in morulae and blastocysts, while centromeric heterochromatin r emains more methylated than that of normal embryos. We propose that the abn ormal time-dependent methylation events spanning the preimplantation develo pment of clones may significantly interfere with the epigenetic reprogrammi ng, contributing to the high incidence of physiological anomalies occurring later during pregnancy or after clone birth.